The effect of orbital-lattice coupling on the electrical resistivity of YBaCuFeO 5 investigated by X-ray absorption

Temperature-dependent X-ray absorption near-edge structures, X-ray linear dichroism (XLD) and extended X-ray absorption fine structure (EXAFS) spectroscopic techniques were used to investigate the valence state, preferred orbital and local atomic structure that significantly affect the electrical and magnetic properties of a single crystal of YBaCuFeO 5 (YBCFO). An onset of increase of resistivity at 180 K, followed by a rapid increase at/below 125 K, is observed. An antiferromagnetic (AFM)-like transition is close to the temperature at which the resistivity starts to increase in the ab -plane and is also observed with strong anisotropy between the ab -plane and the c -axis. The XLD spectra at the Fe L 3,2 -edge revealed a change in Fe 3 d e g holes from the preferential 3d_x^2-y^2 orbital at high temperature (300–150 K) to the 3d_3z^2-r^2 orbital at/below 125 K. The analysis of the Fe K -edge EXAFS data of YBCFO further revealed an unusual increase in the Debye-Waller factor of the nearest-neighbor Fe-O bond length at/below 125 K, suggesting phonon-softening behavior, resulting in the breaking of lattice symmetry, particularly in the ab -plane of Fe-related square pyramids. These findings demonstrate a close correlation between electrical resistivity and coupling of the preferred Fe 3 d orbital with lattice distortion of a single crystal of YBCFO.